Process for preparation of faujasite-type zeolites

ABSTRACT

PROCESS FOR THE PREPARATION OF ZEOLITES WHICH HAVE THE CRYSTAL STRUCTURE OF FAUJASITE AND A COMPOSITION OF THE FORMULA: NA2O.AL2O3.(3.5+ OR -0.5)SIO2.NH2O (N=0 TO 8) BY HYDROTHERMAL CRYSTALLISATION OF REACTION MIXTURES WHICH CONTAIN NA2O, AL2O3, SIO2 AND H2O AT TEMPERATURES OF 20 TO 120* C., CHARACTERISED IN THAT THE COMPOSITION OF THE REACTION MIXTURES, EXPRESSED AS MOLAR RATIOS OF THE OXIDES, LIES WITHIN THE LIMITS OF   SIO2/AL2O3 - 4 TO 7 NA2O/SIO2 - 0.4 TO 0.7 H2O/NA2O - 30 TO 50   THE SIO2 BEING USED IN THE FORM OF ACTIVE SILICIC ACID PRODUCTS WITH SPECIFIC SURFACES OF THE SIO2 PARTICLES OF BETWEEN 150 AND 250 M.2/G. (ACCORDING TO BET) AND THE AL2O3 BEING IN THE FORM OF SODIUM ALUMINATE.

United States Patent 3,685,963 PROCESS FOR PREPARATION OF FAUJASlTE-TYPEZEOLITES Friedrich Schwochow, Leverkusen, Gerhard Heinze,

Schildgen, and Horst Weber, Leverkusen, Germany, assignors toFarbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany No Drawing.Filed Nov. 25, 1969, Ser. No. 879,913 Claims priority, applicationGermany, Dec. 3, 1968, P 18 12 339.2 Int. Cl. C01b 33/28 US. Cl. 423-3294 Claims ABSTRACT OF THE DISCLOSURE Process for the preparation ofzeolites which have the crystal structure of faujasite and a compositionof the formula:

Na O'Al O (3.5i0.5)SiO 'nH O (11:0 to 8) by hydrothermal crystallisationof reaction mixtures which contain Na O, A1 0 Si0 and H 0 attemperatures of 20 to 120 0, characterised in that the composition ofthe reaction mixtures, expressed as molar ratios of the oxides, lies[within the limits of 4 to 7 Na O/SiO 0.4 to 0.7 H o/Na o 30 to 50 theSiO being used in the form of active silicic acid products with specificsurfaces of the SiO particles of between 150 and 250 mP/g. (according toBET) and the A1 0 being in the form of sodium aluminate.

The invention relates to an improved process for the preparation ofsynthetic zeolites having a faujasite structure. Zeolites are generallyunderstood to be a group of crystalline, hydrated aluminosilicates ofmonovalent or higher valent bases which give off their water withoutaltering their crystal structure and are capable of taking up othercompounds instead of the water removed and which furthermore are capableof base exchange (M. H. Hey, Trans. Ceram. Soc. 36 (1937), 84-97). Intheir crystal lattice, there is a rigid three-dimensional primarystructure of 'SiO, and A tetrahedra which are linked together by commonoxygen atoms. Their chemical composition can therefore be represented bythe following general formula:

in which R denotes a metal of valency n or H, NH CH NH etc., X may havea value of from 1.8 to 10 approximately and y a value of from 0 toapproximately 8.

The description of the chemical composition is not sufficient fordefining a specific zeolite since numerous different natural andsynthetic zeolite types are known which differ in their crystalstructure. The X-ray diffraction spec trum is therefore an important andnecessary means for defining a given zeolite type.

The fact that zeolites have found wide acceptance in chemical technologyin the last 10 years as selective adsorption agents and as catalysts isdue to the property of the zeolite structures of containing CEUVlllfiSof constant dimensions which are interconnected by regularly arrangedchannels. These cavities are generally occupied by water molecules.Removal of this zeolitically bound water makes the cavities free to takeup other substances. Naturally, only those molecules which are smallerthan the openings to these cavities can enter the lattice.

The zeolites prepared by the process according to the invention have thecrystal structure of faujasite, a relatively rare mineral which wasfirst described by Damour (Ann. (1. mines (1842), 395). An exactstructural analysis is given by Bergerhoff et a1. (Min. Monatsk. (1958)193.)

Synthetic faujasites, having pore diameters of about 8 to 10 A. belongto the so-called wide pored molecular sieves. In contrast to so-callednarrow pored zeolites (pore diameter 35 A.), they will also adsorbbranched and cyclic hydrocarbons and, when adsorbing smaller molecules,they are distinguished by their high speed of ad sorption.

The increasing demand for wide pored zeolites, mainly in the field ofcatalysts, has led to the development of numerous processes for thesynthesis of faujasite in most recent times. Synthetic faujasites arealso described in the literature by names such as Z 14 Na, Z 14 HS,Zeolite X, Zeolite Y, Zeolite 13 X, Zeolite 10 X etc. The differencesbetween the individual synthetic types and the natural faujasites liemainly in the SiO /Al O' ratio.

The cubical faujasite structure enables this ratio to be varied verywidely within limits of 2 to 6 corresponding to a change in the latticeconstant a0 of from 25.00 A. to 24.60 A. Without any fundamental changesin the crystal structure thereby occurring. Thus, for example, SiO A1 0ratios of 3.0 and 3.9 correspond to lattice constants of (10:24.87 A.,respectively a0=24.77 A. (German Pat. No. 1,098,929, column 4, lines15-17). Two types of synthetic faujasite are distinguished in theliterature according to their SiO content. Synthetic zeolites which havean SiO /Al- O ratio of less than 3 are frequently termed Zeolite X andthose with a ratio greater than 3 are termed Zeolite Y. In thefollowing, the molar SiO content based on 1 mol of A1 0 is chosen as acharacterising feature to identify the synthetic faujasites; a syntheticfaujasite having an SiO /Al O ratio of 3 is therefore termedFaujasite-3. In natural faujasite, the SiO /Al O ratio is also not aconstant factor. In fact, considerable differences. in this ratio werefound in minerals from different deposits. According to their nature ascationic exchangers, both natural and synthetic faujasites may containvarying quantities of different cations.

The known processes for preparing synthetic faujasites consist in ahydrothermal crystallisation of reaction mixtures which contain Na O, A10 SiO and H 0. A distinction may be made between the so-called meltprocesses and so-called precipitation processes, according to whetherthe mixture which is crystallised is prepared via a melt or byprecipitation. The general principle both of the melt process (see e.g.Siedler, Angew. Chem. 22 (1909), 1920) and of the precipitation process(see e.g. Kurnakow, Nachr. d. Akad. d. Wiss. d. UdSSR (1937), 1381) hasin itself been known for a long time.

Due to the large number of structurally different types of zeolite, itis necessary in both processes, if the zeolites are to be prepared inthe pure state, and especially for the preparation of pure faujasite, towork under narrowly defined conditions, the nature of the startingmaterials used, the quantitative proportions, the temperature and thereaction time being critical factors. It has been found,

especially in the industrial production of synthetic faujasite, that therate of gel precipitation, temperature during precipitation and degreeof movement during precipitation or during crystallisation are importantfactors which decisively influence the purity of the products (see e.g.Kerr, J. phys, Chem. 72 (1968), 4, 1385-86). The influence of stirringis of special importance since production on an industrial scale canhardly be carried out without stirring owing to the uneven transmissionof heat in the mixture which forms a sediment.

As described in German patent specification No. 1,138,- 383 thepreparation in the pure state of Zeolite X which is a syntheticfaujasite of the composition on a large scale by the precipitationprocess is possible only if the aluminosilicate gels which come to becrystallised have previously been subjected to ageing at a temperaturerange of from C. to 40 C. Otherwise, the gels used are so sensitive tostirring that most of the product formed is not Zeolite X butPhillipsite which is not wanted as an industrial product. According toGerman Pat. No. 1,164,384, much purer products are also obtained in thesynthesis of Zeolite Y, a synthetic faujasite richer in SiO which hasthe composition if the mixture of starting materials has first beendigested at the temperature of the surroundings.

Another disadvantage of the known processes is that a very large excessof SiO has to be used in the reaction mixture, which is subsequentlylost with the mother liquid. Thus in Example 4 of German Pat. No.1,098,929, Zeolite Y with SiO /Al O =3.29 is obtained from a mixture inwhich SiO /Al O :l2, and in Example 5 Zeolite Y with SiO /Al O =3.4 isobtained from a mixture in which The present invention relates to aprocess for the preparation of synthetic zeolites which have the crystalstructure of faujasite and a composition corresponding to the generalformula: Na O.Al O .(3.5i0.5)SiO .nH O (11:0 to 8) by hydrothermalcrystallisation of reaction mixtures which contain Na O, A1 0 SiO and H0 at temperatures of C. to 120 C., which is characterised in that thecomposition of the reaction mixtures, expressed as molar ratios of theoxides, lies within the limits of SiO /Al O =4 to 7, Na O/SiO -=0.4 to0.7 and H O/Na O= to 50, the SiO being used in the form of activesilicic acid products with specific surfaces of the SiO content of 40%by weight. Sols having SiO contents BET), and the A1 0 used being in theform of sodium aluminate.

The process according to the invention overcomes the above describeddisadvantages of the known processes and provides an industrially andeconomically especially advantageous method of preparation with highvolume/ time yields. It enables synthetic faujasites to be prepared fromreaction mixtures with a low SiO excess and moreover the reactionmixtures can be stirred during the crystallisation. Time consuming agingof the reaction mixture is not required. As soon as the reactionmixtures have been prepared, it can be heated with stirring to theoptimum temperature for accelerating crystallisation without unwantedby-products being formed.

To carry out the process according to the invention, aqueous sodiumaluminosilicate gels are prepared at the surrounding temperature i.e. attemperatures of from 15 C. to C., from silica sols or from silicic acidfillers or from mixtures of the two aluminate solutions and optionallysodium hydroxide solutions and water, the total concentration of themixtures of starting materials lying within the limits indicated above.

Silicic acid sols which have specific surfaces of the SiO particlesaccording to BET of 150 to 250 mP/g. are suitable for the processaccording to the invention. Such silicic acid sols are advantageouslyprepared by ion exchange treatment of dilute waterglass solutionsfollowed by alkaline stabilisation. They have an exceptionally lowconcentration of foreign bodies and are therefore astonishingly stableso that, for example, they can be concentrated by evaporation atatmospheric pressure to a $10 content of 40% by weight. Sols having SiOcontents of 20 to 40% by weight are generally used for the process ofthe invention. Processes for the preparation of silicic acid sols aredescribed e.g. in U.S. patent specifications Nos. 2,244,325 and2,631,134 and in German patent specification No. 1,026,735.

By silicic acid fillers are meant large surfaced synthetic silicic acidswhich have been obtained by precipitation from an alkali metal silicatesolution and which in contrast to silicic acid gels have a fiocculentsecondary structure. In the preparation of these silicic acid fillers,the primary particle size and hence the specific surface can becontrolled within wide limits by the concentration and temperature ofthe solution and by the speed of precipitation, vigorousness ofstirring, etc. For the process of the invention, it is suitable to usesilicic acid fillers which have specific surfaces of the SiO particlesaccording to BET of from 150 to 200 m. g. Processes for the preparationof suitable silicic acid fillers are described e.g. in German patentspecification No. 1,023,022.

Within the scope of the invention, the above-described silicic acid solsand silicic acid fillers which have the large specific surfacesmentioned above are considered as active silicic acid products.

Determination of the specific surface of the SiO particles in silicicacid sols or in silicic acid fillers is carried out by the method ofBrunauer, Emmet and Teller (BET) by measuring the nitrogen adsorption.The silicic acid fillers may be used directly for the measurement;aqueous silicic acid sols must first be dried at 105 C. after theaddition of acid to adjust the pH to about 5.

The sodium aluminate solution is preferably prepared by dissolvingaluminium oxide trihydrate (hydrogillite) in a 45% sodium hydroxidesolution followed by dilution to the concentration required for thereaction mixture. Alternatively, commercial solid crystalline sodiumaluminate, for example of the composition 1.25

may be directly dissolved in water or in a sodium hydroxide solution.

The precipitates or suspensions, after they have been renderedhomogeneous at the surrounding temperature, are heated with stirring tothe optimum crystallisation temperature of to 100 C., preferably to C.,and crystallised at this temperature for a time sufficient to effectcrystallisation. Normally at least 4 hours are necessary, more thanabout 48 hours being possible but not economic. Preferably 12 to 24hours give excellent results, with or without stirring of the reactionmixture. Lower crystallisation temperatures may also be employed butthese are uneconomical because the crystallisation time then increasesconsiderably. The crystalline paste separated from the mother liquor iswashed with distilled water until the pH is 9 to 10 and then dried andoptionally activated.

An additional advance of the process according to the invention over theknown process lies in the fact that, in the syntheses, within the rangeof concentrations claimed, the 'SiO /Al O ratio in the reaction productscan be controlled within narrow limits. The results of the experimentsrepresented in Table 1 show the influence of the SiO /Al O or Na- O/SiOratio in the original mixture on the SiO content of the faujasitesobtained from them. As will be described more fully in the examples,sodium aluminosilicate gels with a constant H O/Na O ratio (=30) wereprecipitated from aqueous silica sols (30% SiO BET 200 m. /g.), sodiumaluminate solutions, sodium hydroxide solutions and water for thispurpose and crystallised under the same conditions at 85 C. in thecourse of 24 hours. As demonstrated by the SiO /Al O ratios of thepreparations shown in the last column of Table 1, by suitably varyingthe starting mixtures one can control the SiO /Al O ratio in thepreparation to obtain practically any desired value within the limits of3 to 4, i.e. a series of faujasite-3 to faujasite-4.

TABLE 1 Dependence of the Slog/A1203 ratio in the faujasite upon theSiOg/AhO; ratio in the reaction mixture (conditions: 24 hours 85 0., HO/Na O 30, source of S1012 silica sol, BET=200 m./g., 30% S102)Determination of the SiO /Al O ratio in crystalline faujasites isadvantageously carried out by determining the lattice constants a Thevalues obtained for this ratio represent the quantities of Si actuallybuilt into the zeolite lattice whereas in chemical analysis higherratios are often found if in addition to crystalline zeolite someamorphous Si0 not built into the lattice is also present.

The a values of synthetic faujasites are obtained from the equationwhich generally applies to cubical crystals:

in which d is the lattice plane distance obtained from theDebye-Scherrer diagrams and h, k and l are the corresponding Millerindices.

According to D. W. Breck and E. M. Flanigen (Synthesis and Properties ofUnion Carbide Zeolites L, X and Y, Paper read at the conference onMolecular Sieves, London, 4th-6th April, 1967), and a value and the SiO/Al O ratio are related to each other as follows:

In all the faujasites prepared by the process according to theinvention, the SiO /Al O ratio was calculated according to the aboveformula from a values which were determined by X-ray analysis.

The experiments of Table 2 illustrate the influence of crystallisationtime and crystallisation temperature. According to these results, thetime required for complete crystallisation at 75 C. is about 48 hours,at 85 C. about 16 hours and at 100 C. only about 8 hours. The increasein crystallisation time beyond these minimum times causes practically nochange in the preparations at a given temperature.

The influence of the crystallisation temperature on the SiO;. content ofthe preparations is also slight. Lower temperatures however, appear toresult in a slightly higher SiO /Al O ratio than higher temperatures. Onthe whole, all the experiments of Table 2 illustrate the goodreproducibility of the SiO A1 0 ratio in the synthesis from reactionmixtures of identical initial concentrations.

TABLE 2 Influence of crystallisation temperature and crystallisationtime [Conditionsz S102 source: silica sol (30% S102, BET 200); sodiumaluminate solution: 1.7Na2O/AlzO molar ratios: Si02/Al203=5, Na2OISiO2=0.5, HzQ/NazO =30] Crystal- Crystallisation lisation H2O uptakeSiOz/AlaO 3 temperature time (25 0.,10 mm. (calculated C (hours) Hg)(percent) an (A.) from (In) 15 24 14.6 O) 75 48 32. 1 2A. 810 3. 40 7572 32. 4 24. 802 3. 47 75 913 31. 3 24. 810 3. 40 s5 8 7. 9 16 32. 424.810 3. 40 85 24 32. 6 24. 818 3. 31 85 48 31. l 24. 811 3. 39 85 7232. 7 24. 816 3. 35 8 32. 3 24. 816 3, 35 100 16 31. 6 24.816 3. 35 10024 31. 9 24.816 3. 35 100 48 31. 5 24.816 3. 35

1 Mixture of iaujasite and amorphous substances.

TABLE 3 Influence of the S102 starting material [ConditionszsiO2/Al203=6, Na2O/SiO2=0.6, H2O/NazO=30; A1 03 source: Na aluminatesolution (1.7 NagO/AIQO crystallisation: 24 hours at 85 (3.]

1 In g./100 g. (25 0., 10 mm. Hg). 2 Silica gel, narrow pored, ground,18.6% H20; suppliers: G br. Herrmann, KolnEhrenield).

Experiments with different SiO starting materials are summarised inTable 3. They show the superiority of the sources of Si0 proposed forthe process.

It is surprising that the use of a solid silicic acid filler as usede.g. in the rubber industry as a reinforcing filler provides the sameadvantageous results as the use of an aqueous silicic acid sol although,in the first case, a solid is reacted with an aluminate solution to formthe zeolite whereas in the second case, a homogeneous gel isprecipitated from soluble components and then crystallised. The use ofsilicic acid fillers precipitated in the wet, moreover, leads underotherwise identical conditions to slightly lower SiO /Al O ratios thanthe use of aqueous silicic acid sols. Thus, for example, from a reactionmixture which has the molar ratios and H O/Na O=40 one obtains at 85 C.pure faujasite zeolite with a lattice constant of a =24.826 A.,corresponding to an SiO /Al O ratio of 3.25 when using a wetprecipitated silicic acid filler with a BET value of g./m. whereas, whenusing an aqueous silicic acid sol under otherwise identical conditions,one obtains a slightly lower lattice constant of a =24.810 A. and thus aslightly higher SiO /A1 -O ratio of 3.40.

The SiO content in the crystalline product evidently depends on thespecific surface of the Si0 particles in the starting material since thesilicic acid filler which has a BET value of 180 m. /g. has a slightlylower specific surface of SiO particles than the silicic acid sol usedwhich has a BET value of 200 m. /g. On the other hand, the use of otherSi0 sources, which is not an object of the invention, e.g. the use ofuncalcined kieselguhr which has a BET value of 28 m. g. for the specificsurface of the SiO particles, does not lead to pure faujasite under theconditions according to the invention.

Aqueous silica sol (BET 200) 6 mol of SiO'i/litre.

Density 1.2 46.6 mol of HgO/litre. \Vct precipitated silicic acid fillerAmorphous SlOz containing 11.6%

(BET 180). of H20.

Sodium aluminate solution 2 mols of AlzOg/litre.

Density 1.36 3.4 moles of NazO/litre; 52 mol of Sodium hydroxidesolution (45%)... 8.4 mol of Naz lltre.

Density 1.48 53.4 mol HzO/lltIO.

EXAMPLE 1 A solution of 100 ml. of aluminate, 19.2 ml. of sodiumhydroxide solution and 30 ml. of H 0 was precipitated with 167 ml. ofsilicic acid sol at room temperature (SiO /Al O =5; Na O/SiO =0.5; HO/Na O=30). The homogeneous gel was heated to 75 C. with stirring inabout 2 hours and crystallised at this temperature for 48 hours withoutstirring care being taken to avoid loss of water. After cooling to roomtemperature, the crystalline paste was filtered off and washed withdistilled water until the pH was 9. The water uptake capacity of thepreparation which was activated at 500 C. was 32.1 g./100 g. (25 C., mm.Hg). According to X-ray analysis, pure faujasite-3.40 with a latticeconstant of a =24.8l0 A. corresponding to a ratio of SiO /Al O =3.40 wasobtained.

EXAMPLE 2 A gel of the composition 3.6Na O.Al O .6SiO .l80H O wasprecipitated at room temperature from 100 ml. of aluminate solution, 200ml. of aqueous silica sol, 45.7 ml. of sodium hydroxide solution and 340ml. of water. After homogenisation, the gel was heated to 85 C. withstirring and crystallised at this temperature in 24 hours withoutstirring. When the products had been washed out and activated, it showeda water uptake capacity of 32.0 g./ 100 g. (25 C., 10 mm. Hg) andaccording to X-ray analysis it was pure faujasite-3.52 with SiO /Al O=3.52 ((1 24.796 A.).

EXAMPLE 3 In this example, the process was carried out in concentratedsolution without the addition of water. A gel of the composition 2.58NaO.Al O .6SiO .77.5H O' prepared from 100 ml. of aluminate solution, 200ml. of silica sol and 20.5 ml. of sodium hydroxide solution was heatedto 85 C. with stirring and crystallised at this temperature for 24 hourswithout stirring. The washed and activated product showed a water uptakecapacity of 30.7 g./100 g. (25 C., 10 mm. Hg) and had a lattice constantof a =24.736 A. (corresponding to SiO /Al O =4.13), i.e. faujasite-4.13.

EXAMPLE 4 The following example shows that pure faujasite is obtained bythe process according to the invention even with continuous stirring.

A gel of the composition 3.6Na O.Al O .6SiO .l10H O which had beenprecipitated at room temperature was heated to 100 C. with stirring andcrystallised at 100 C. in 8 hours with continuous stirring. Aftercooling to room temperature, the resulting crystalline paste wasfiltered off, washed with distilled water and activated at 500 C. The

water uptake capacity of the preparation was 32.2 g./l00 g. (25 C., 10mm. Hg). According to X-ray analysis, pure faujasite-3.32 with a=24..818 A. or SiO Al O =3.32 was obtained.

In the following examples, the Si0 starting material used consistedpartly or entirely of a wet-precipitated silicic acid filler with BET of180 g./m.

EXAMPLE 5 81.4 g. of silicic acid filler were introduced with vigorousstirring at room temperature into a mixture of 100 ml. of aluminate,45.8 ml. of sodium hydroxide solution and 380 ml. of water. Thesuspension of the composition 3.6Na O.Al O .6SiO .144H O was heated toC. in about 2 hours with constant stirring. The crystallisation time at85 C. was 48 hours. The preparation did not differ in purity from thatprepared from a silicic acid sol. The water uptake capacity was 32.5 g./g. (25 C., 10 mm. Hg). X-ray analysis revealed a lattice constant of a=24.826 A. corresponding to a ratio of SiO /Al O 3.25, in other wordsfaujasite-3.25.

EXAMPLE 6 This experiment was carried out with a silicic acid filler ina manner completely analogous to Example 5. To increase the Si0 contentin the reaction product, however, a gel of the composition prepared from100 ml. of aluminate solution, 95.2 g. of silicic acid filler, 43.4 ml.of sodium hydroxide solution and 356 ml. of water was used. The reactionproduct consisted of pure faujasite-3.83 with a lattice constant of a=24.763 A. (corresponding to 3.83SiO /Al- O and a water uptake capacityof 31.6 g./100 g. (25 C., 10 mm.

EXAMPLE 7 In this example, a mixture of silica sol and silicic acidfiller was used as a source of SiO 40.8 g. of silicic acid filler werefirst introduced into 100 ml. of aluminate solution with 45.6 ml. ofsodium hydroxide solution and 226 ml. of water, and 100 ml. of silicasol were then added with stirring. This mixture corresponds to a molarratio of SiO /Al O =6; Na 0/SiO =0.6; H O/NaO=35. The reaction mixturewas heated to 85 C. with stirring and crystallised at this temperaturein 48 hours. The product obtained consisted of pure faujasite-3.3 andshowed a gager uptake capacity of 32.3 g./ 100 g. (25 C., 10 mm.

EXAMPLE 8 This experiment gives an example for carrying out the processaccording to the invention on a larger scale. The following startingmaterials were used:

Sodium aluminate solution: 194.3 g. of Al O /litre; 196.4

g. of Na O/Iitre; density: 1.34

Aqueous silicic acid sol (BET 200 m. /g.): 30% of SiO:,

density: 1.2

Colnggrgtrated sodium hydroxide solution: 50%, density 26.25 litres ofthe aluminate solution, 8.6 litres of sodium hydroxide solution and 14.5litres of water were introduced into an electrically heatable, litrevessel equipped with a stirrer and efficient heat isolation. 50 litresof the silicic acid sol were added to this solution at room temperaturewith constant stirring. The precipitated gel which had the composition,3.3Na O.Al O .6SiO .100H O was heated to 90 C. in three hours withconstant stirring. Heating and stirring were then stopped and thereaction mixture Was left to stand for 18 hours. The temperature duringthis time fell to about 75 C. The reaction mixture was decanted withwater several times, and the crystal paste was separated from the motherliquor through a filter press and washed out on the press (3 hours).After the product had been blown out to be dried, it was finally driedat 100 C. in a well ventilated oven.

What is claimed is:

1. Process for the preparation of Zeolites which have the crystalstructure of faujasite and a composition of the formula: U

Na O.Al O (3.5 :0.5 SiO .nH O

(n= to 8) by hydrothermal crystallization of reaction mixtures whichcontain Na O, A1 0 SiO and H 0 at temperatures of 20 to 120 0,characterised in that the composition of the reaction mixtures,expressed as molar ratios of the oxides, lies within the limits of 4 t07 Na O/ S102 0.4 to 0.7 H O/Na 0 30 to 50 the SiO being used in theformof active precipitated silicic acid filler with specific surfaces of theSi0 particles 10 of between 150 and 250 mfl/ g. (according to BET) andthe A1 0 being in the form of sodium aluminate.

2. Process according to claim 1, wherein the reaction mixture is stirredwhile it is being heated up and during the crystallization.

3. Process according to claim 1, wherein the hydrothermalcrystallization is effected by steps comprising preparing an aqueoussodium aluminosilicate gel at the surrounding temperature andimmediately thereafter stirring and heating to l00 C. and heating insaid temperature range for 448 hours.

4. Process according to claim 3, wherein the reaction mixture is stirredwhile it is being heated up and during the crystallization.

References Cited UNITED STATES PATENTS 3,130,007 4/1964 Breck 23-1 133,343,913 9/1967 Bobson 23-113 2,940,830 6/ 1960 Thornhill 23-1 823,310,373 3/1967 Johnson 231l2 3,058,805 10/ 1962 Weber 23l13 EDWARD J.MEROS, Primary Examiner US. Cl. X.R. 423-339

